Various methods for forming honeycomb structures have been proposed up to now. However, the honeycomb structures used as catalysts, whose core length is exceedingly longer than the core cross section, have normally been formed by extrusion molding using dies. Accordingly, it is safe to say that the shapes and structures of the dies constitute very important factors for obtaining honeycomb structures. Generally speaking, the die used for extrusion molding is arranged to form a lattice molding passage in the outlet surface of the die, and further to form a supply passage that is opposite to the intersecting portion of said molding passage in the inlet surface of the die opposite to said outlet surface and has a depth penetrating to the bottom of said molding passage. When carrying out extrusion molding by the use of the thus constructed die, if the plastic material to be extrusion molded is not arranged to pass through the lattice molding passage at a substantially uniform speed, there are caused defects such as distortion of the resulting honeycomb structure, blur inside the partition wall, partial vanishing of the lattice (which is called "hanging" hereinafter) and the like.
Accordingly, in order that the plastic material to be extrusion molded may be extruded at a uniform speed from every portion of the molding passage, there have been proposed various methods designed to form a through hole along the lattice at the place connecting the supply passage with the molding groove of the die.
The typical die of this sort has been disclosed in Japanese Published Utility Model Gazette No. 3780/1982 Specification, which is shown as the prior art in FIG. 1. In this drawing, the reference numeral 21 stands for a die, wherein a lattice molding passage 22 is formed in its outlet surface 23; plural and independent supply passages 25 are formed in a raw material inlet surface 24 opposite thereto, said passages being opposite to the intersecting portions of said molding passage 22 and extending toward the outlet surface 23; and a large number of independent circular holes 26, whose diameters is larger than the passage thickness, penetrate from the sides of the die along the rear end of the molding passage 22 so as to cross each other.
In Japanese Laid Open Patent Gazette No. 178712/1982 Specification, furthermore, there is disclosed a die for molding a honeycomb structure characterized by the feature that in order to prevent the hanging phenomenon of the honeycomb structure to be produced, the through hole of the die takes the form of an oblong or a rectangle whose long diameter/short diameter ratio is 1.2-5 and further the ratio of the short diameter of the through hole to width of the lattice cut-in passage is arranged to be in the range of 1.2-10.
In Japanese Published Patent Gazette No. 164/1986 Specification, still further, there is disclosed a die characterized by the feature that a guide groove corresponding to the through hole is formed, the cross-sectional opening area of said guide groove is larger than that of a molding passage, and further a resistance portion guide groove is formed corresponding to a supply passage portion.
However, these prior art dies (apparatuses) for molding honeycomb structures can not necessarily obtain moldings that satisfy the requirements of extrusion molding ability, the properties of the obtained honeycomb structures such, for instance, as distortion, hanging and mechanical strength, and the properties such as cracking and the like occurring at the time of drying and calcining, depending upon the properties of plastic materials to be extrusion molded.
Especially when extrusion molding a honeycomb structure raw material consisting mainly of titanium oxide and/or silica-alumina and a fibrous substance-containing plastic material that exhibits a water-releasing phenomenon when pressurized and deteriorates its fluidity, it was difficult for the usual molding dies to maintain a good molding ability because masses caused by a partial water-releasing phenomenon within the through hole block up part of the molding passage, the fibrous substance clings to part of the molding passage to thereby form a non-flowing section, the resulting honeycomb structure is subject to hanging, and the extruding pressure increases gradually in the course of molding.
The object of this invention is to provide a die for molding a honeycomb structure suitable for extrusion molding a plastic material containing a fibrous substance that exhibits a water-releasing phenomenon when pressurized and deteriorates its flow.